如何在Swift中为WKWebview制作透明背景

Question

使用Swift5，

我尝试将WKWebview的背景设置为透明。

不幸的是，现在，在Web视图中显示.pdf文件时，颜色始终为灰色！但是我想将其更改为透明。我该怎么办？

我使用以下代码：

class SomeViewController: UIViewController {

    @IBOutlet weak var webView: WKWebView!

    var document: Document!
    var documentName: String!

    override func viewDidLoad() {
        super.viewDidLoad()

        webView.navigationDelegate = self

        // I try this - but it does not work...
        // WHY WHY WHY WHY ???????????????????????
        webView.isOpaque = false
        webView.backgroundColor = UIColor.clear
        webView.scrollView.backgroundColor = UIColor.clear

        BusyView.show()

        // here the .pdf file gets loaded
        // the file is loaded correctly but its background is still gray (instead of transparent)...
        if let fullURL = NSURL.fileURL(withPathComponents: [NSTemporaryDirectory(), documentName]) {
            let request = URLRequest(url: fullURL)
            webView.load(request)
        }

        navigationItem.title = document?.title
        navigationItem.largeTitleDisplayMode = .never
    }
    // ....
}

extension MailboxViewDocumentController: WKNavigationDelegate {

    func webView(_ webView: WKWebView, didFinish navigation: WKNavigation!) {
        BusyView.hide()
        scrollToDistanceFromUnderneathBlurredTopBar()
    }

    fileprivate func scrollToDistanceFromUnderneathBlurredTopBar() {
        var scrollDistance: CGFloat
        if #available(iOS 11, *) {
            scrollDistance = view.safeAreaInsets.top
        } else {
            scrollDistance = topLayoutGuide.length
        }
        DispatchQueue.main.asyncAfter(deadline: .now() + .milliseconds(10)) {
            let scrollPoint = CGPoint(x: 0, y: -scrollDistance)
            self.webView.scrollView.setContentOffset(scrollPoint, animated: false)
        }
    }

    func webView(_ webView: UIWebView, didFailLoadWithError error: Error) {
        BusyView.hide()
    }
}